In general, in an ultrasonic diagnostic apparatus as a subject information obtaining apparatus, the spatial resolution in the depth direction when image data is formed by a pulse-echo method can be represented by an expression (nλ)/2, where λ denotes the wavelength of an ultrasonic wave and n denotes the number of waves transmitted. For example, when two wavelengths of an ultrasonic wave having a center frequency of 12 MHz are transmitted, the spatial resolution in the depth direction is about 0.13 mm.
The pulse-echo method will be described. First, when an ultrasonic pulse (an elastic wave) has been transmitted to a subject, an ultrasonic wave is reflected and comes back in accordance with differences in acoustic impedance inside the subject. Next, the reflected wave is received and image data is generated using a received signal of the reflected wave. Typically, an envelope of the received signal is obtained and converted into values of brightness, in order to generate the image data. By repeating transmission and reception of an ultrasonic wave in a plurality of directions or positions in the subject, brightness information on a plurality of scan lines in a direction in which the ultrasonic waves have been transmitted and received can be obtained. By arranging the brightness information on the plurality of scan lines, the inside of the subject can be imaged.
In general, in the ultrasonic diagnostic apparatus, a plurality of conversion elements that convert ultrasonic waves into electrical signals are used and time differences are provided between the waveforms of signals received by the conversion elements, so that the inside of the subject is focused both in the transmission and the reception.
Although it is possible to realize a spatial resolution of about 0.13 mm in the depth direction by using the pulse-echo method, higher spatial resolution is required. For example, if the layer structure of the blood vessel walls of a carotid artery can be observed in more detail, it is possible to contribute to early detection of arteriosclerosis or the like.
As techniques for improving the spatial resolution in the depth direction, a frequency-domain interferometry (FDI) method and a Capon method, which is a type of adaptive signal processing, are used in NPL 1, in order to present results of imaging of the layer structure of blood vessel walls. By using the FDI method and the Capon method for received signals, it is possible to further improve the spatial resolution in the depth direction (scan line direction). However, a plurality of reflection layers are supposed to exist in a range (processing range) of a signal in the depth direction that has been cut out in order to execute the processing of the FDI method. In addition, it is likely that a plurality of waves reflected from reflection layers that are located close to one another have a high correlation. It is known that if the adaptive signal processing such as the Capon method is directly adopted for received signals of a plurality of such reflected waves that have a high correlation, unexpected effects such as cancellation of a desired signal can be produced. The effects produced by signals (coherent interference waves) that have a correlation can be reduced by using a frequency-averaging technique, and the FDI method and the Capon method can be adopted for the received signals of reflected waves.
Furthermore, when the frequency-averaging technique is adopted for a received signal of an elastic wave having a wide frequency band, such as a pulse wave, whitening of the received signal is performed using a reference signal. In PTL 1, an apparatus is described in which a plurality of standard signals for forming a reference signal are combined using a certain interpolation ratio and an obtained signal (an operational reference signal) is used as the reference signal.